Cyclic (one per rev sinusoidal) blade feathering of helicopter rotor blades provides for attitude control of the helicopter. Higher harmonic blade feathering or nonharmonic localized blade feathering (hereinafter jointly referred to an antivibratory blade feathering) is known to be capable of reducing rotor-induced vibration. Typically, antivibratory blade feathering is superimposed on the cyclic blade feathering through rapid, dynamic inputs to the control servos acting on the swashplate. The primary disadvantage of such techniques resides in the decreased reliability due to increased vibratory loads and motions in the rotor control system, the increased hydraulic or electric power requirement for the control servos, and the weight and cost of the engineering change to correct for these problems.
Some helicopter main rotor blades may have a built-in twist to enhance hover performance. The built-in twist may be thought of as a varying angle of attack for the blade, decreasing (for instance) from ZERO degrees at the blade root to MINUS SIXTEEN degrees at the blade tip. Having a high built-in twist which optimizes hover performance is not compatible with the need for less twist to optimize performance in forward flight, and will increase vibratory air loads.